In many process industries direct fired heaters are commonly used to heat gaseous and liquid fluids, such as distillation column feeds and reactor feeds. The direct fired heater generally includes an enclosed structure and a conduit through which the process fluid enters the structure, passes within the structure, and exits the structure. Necessary heat is obtained from the combustion of liquid or gaseous fuel using air-fired burners that fire into the structure Heat of combustion passes to the process fluid through the walls of the conduit, which often takes a coiled or otherwise elongated path within the structure so as to increase the opportunity for this heat transfer.
The combustion reactions, however also form NOx (by which is meant oxides of nitrogen such as but not limited to NO, NO2, NO3, N2O, N2O3, N2O4, N3O4, and mixtures thereof). As NOx is an environmental pollutant, it would be very desirable to lessen the amount of NOx that is generated in the operation of process heaters. However, attempts to reduce the formation of NOx are frustrated by the other constraints present in process heater operation, such as the heat uptake limitations imposed by the metallurgical properties of the material from which the conduit is formed, the tendency of the process fluid to experience coking, and the ability to maintain negative draft in the heater that is adequate to carry combustion air into the heater. These frustrations are compounded by the generally accepted understanding that combustion of fuel with oxygen alone, or with oxidant having an oxygen content elevated over that of air, is expected to increase the temperature of the flame and therefore increase the amount of NOx that is formed.
Operation of air-fired process heaters also presents challenges of obtaining satisfactory heat transfer to the process fluid without exceeding the maximum temperatures that can be tolerated to avoid coking of the process fluid and metallurgical damage to the conduit. It has generally been understood that within the radiant section of the process heater there is a heat flux gradient along the length of the burner flames. The gradient is substantial enough that remaining below the maximum tolerable temperature at peak heat flux locations forces acceptance of a less than maximum overall average heat flux to the process fluid. This in turn imposes constraints on the throughput that can be attained, on the maximum absorbed duty, or on the maximum outlet temperature of the process fluid.
The present invention achieves the objective of lessened NOx formation, while achieving greater uptake of the generated heat and providing other advantages described herein.